The invention relates to an aerodynamic measurement probe of an airstream along a wall. The invention makes it possible notably to determine the attack of an airstream relative to an axial reference tangential to the wall. The invention is particularly useful in the aeronautical field where knowledge of the attack of an airstream surrounding an aircraft is essential to the piloting of the aircraft. The attack relative to a horizontal plane is an important parameter for determining the buoyancy of the aircraft for example in a landing. The attack relative to a vertical plane is also important representing the side slip of the aircraft. To determine these two parameters, attack and side slip, it is possible to locally measure the orientation of the airstream relative to the skin of the aircraft. This involves local attack measurements that are performed at particular points of the aircraft. It is also possible to measure two components of the speed of the airstream at a point of the skin of the aircraft to determine the direction of the airstream.
Numerous sound principles are known and commonly used for speed or flow direction measurements.
A first probe family uses an appendage extending from the skin of the aircraft. This appendage can be fixed. It can include air pressure taps around this appendage or sensors measuring the force exerted by the airstream. This appendage can be mobile in the form of a flag being oriented in the axis of the airstream. The orientation of the flag then gives the attack of the airstream. The first probe family also contains the use of the vortex effect by performing measurements on vortices downstream of a cylindrical body for example, the use of windmills to measure a flow speed in a given direction, the use of a hot wire as anemometer.
The probes of this first family are fragile because of the presence of a body external to the skin of the aircraft. They have to be designed to withstand the abrasion of the airstream and above all of the particles that it can carry. They hamper the aerodynamism of the aircraft by generating a drag. In flights at high altitude, these probes must be de-iced and therefore consume electrical power. The mobile probes must include sealing systems between fixed and mobile parts with the minimum of friction.
A second probe family makes it possible to eliminate any appendage external to the skin of the aircraft. Optical systems organized around lasers exist, but they are currently complex, expensive, bulky. They are still generally used as reference systems.
There is therefore great interest in ultrasound systems among numerous manufacturers. The basic idea is to measure the propagation times of acoustic waves between emitting elements and receiving elements fixed relative to each other, to identify the speeds of a sound wave, as a function of the speed of the sound and the speed of the fluid, in several directions, and finally the direction of flow, for example the attack or side slip in aeronautical applications.
Several types of device are currently known. In document U.S. Pat. No. 4,143,548: an emitter generates an ultrasound wave towards two receivers respectively situated upstream and downstream. The phase relating to the signals received gives an indication on the difference in the speeds in each of the two propagation directions, towards upstream and towards downstream. This device implicitly assumes that the signals are not disturbed so that it is possible to measure the phase between two sinusoidal signals. Moreover, this device imposes constraints on the frequency or wavelength with respect to the distance between emitter and receivers. Finally, the receivers must be identical in terms of transfer function and intrinsic delay.
The documents U.S. Pat. No. 4,112,756 and U.S. Pat. No. 4,890,488 propose similar ideas, with propagation time measurements between emitters and receivers according to different configurations.
The document U.S. Pat. No. 5,585,557 proposes an entirely passive device, in other words without emitter. Turbulences in the flow are received and detected by a first receiver and are propagated downstream, where they are received and detected by other receivers situated at one and the same distance, with delays dependent on the characteristics of the flow in direction and speed. The transit times are calculated from crossed correlation calculations between the signals. An estimate of the direction of flow is that which is defined by the first receiver and the receiver situated downstream presenting the shortest transit time. The accuracy of the system is associated with the number of sensors.
The document U.S. Pat. No. 7,155,969 proposes an enhancement and a simplification of the foregoing, using a smaller number of sensors, and capable of operating through the skin of the airplane. These sensors are not necessarily microphone-type acoustic sensors that require passages in the skin of the airplane to detect the pressure fluctuations, but could also be accelerometers, strain gauges or other sensors mounted on the skin of the airplane for example. The propagation of the pressure fluctuations generated by the turbulence can be replaced by the propagation of a mechanical excitation of the skin of the airplane by means of an appropriate device, active piezoelectric sensor for example. The propagation time measurements are also made from crossed correlation calculations between the signals received by the different sensors.
All these documents describe systems comprising several sensors receiving signals, and deviations between the received signals are used to work back to the propagation times, then to the speeds, and finally to the direction of flow.
The experimental measurement of the travel time between an emitter and a receiver may prove greater than that provided by the theory, to within a relatively constant value. Consequently, the methods of measuring travel times require calibrations of the receivers, each having its own characteristics in terms of response time, bandwidth, etc. These calibrations of the receivers are dependent on the environmental conditions, temperature and pressure in particular. The precise measurement of the attack based on travel time measurements on the ultrasound waves therefore proves fairly complex, because of the characteristics of the different receivers. In the case of totally passive systems, with no active exciter, the acoustic signals received are simply an acoustic noise. A malfunction of one of the receiving sensors is thus difficult to detect, unless, perhaps, the sensor is totally short circuited. Also, the crossed correlation calculations necessitate samplings and storage of low level signals to be able to be carried out.
Finally, all these systems implicitly assume that the signal of acoustic noise type is propagated identically to itself, which is only a first approximation.